FIG. 2 of DE 103 45 089 A1 (not reproduced here) discloses a generic structural assembly comprising a tappet and a pump piston which bears against an inner side of a bottom of the tappet through an annular collar which widens its outer peripheral wall. An annular groove adjoining the annular collar (in direction of the pump piston) is surrounded with radial lash by a bore of a spring plate bearing against the annular collar. It can be seen in the figure that the spring plate extends axially spaced from the inner side of the bottom of the piston and that a coil compression spring serving for resetting the pump piston (intake stroke) acts against a bottom-distal side of the spring plate.
In the aforesaid embodiment, the connection of the spring plate to the pump piston with radial lash leads to an at least partial uncoupling of transverse forces on the pump piston, which transverse forces cause reaction forces in the guide of the piston pump and, in particular if the lubrication conditions are unfavorable (e.g. fuel lubrication), to premature wear and higher actuation forces. The aforesaid transverse forces originate, for example, in the spring motion (torsion) and in component tolerances or other installation effects.
It is remarked that a production of the pump piston with an annular collar is too expensive and complex for large series manufacturing. For example, complex chip removal is required and the annular groove must be subsequently finely machined separately from the fine machining of the rest of the smooth cylindrical part of the pump piston. Besides this, it is questionable whether the spring plate, merely seated on the pump piston, is capable of supporting the resetting forces occurring during operation without being damaged or whether it has to be made with unnecessarily solid dimensions. In addition, the coil compression spring has only an inadequate guidance in the bearing region. As a result, when compressed (pump lift), the spring may contact the guide skirt, in an undesired manner, on the outside. Finally, due to the relatively small surfaces of the spring plate and collar in contact with each other, there exists the danger of a “digging-in”, so that, under certain circumstances, transverse forces would indeed be introduced into the pump piston.